Some Definitions Igneous refers to processes Rock Cycle Volcanoes and rocks involving or formed from molten rock. Igneous rocks are one of the three main rock types, the others being I Types of Volcanoes sedimentary and metamorphic rock. Igneous rocks are II Plutonic Activity formed by the cooling and III Plate Tectonics and Igneous Activity solidification of magma or IV Magma and Lava Magma is molten rock, and contains volatiles (gases such USGS V Types of Lava Flows as water and CO2) and solids (crystals and rock fragments). Magma is found beneath the Earth’s surface. Lava is magma that has extruded onto the Earth’s surface and is the volcanic equivalent of magma. 1

I Types of Volcanoes Shield Volcanoes Shield volcanoes form from the accumulation of very fluid () Successive eruptions build up mountainous accumulations of material flows. They can be very large and have a broad and slightly domed forming a . shape. There are 3 main types of volcanoes: The Hawaiian islands are formed from shield volcanoes. Mauna Loa on 1. shield the island of is nearly 6 miles in height (see photo). 2. composite They differ in size and composition. Kilauea has been continuously erupting since 1983. 3.

Mauna Loa, Hawaii Medicine Lake Volcano, CA Good examples of shield volcanoes are found in the Hawaiian islands - Kilauea, Mauna Loa, Mauna Kea. Small shield volcanoes, such as Medicine Lake, are found in . Kilauea Shield volcanoes are found on Eruption of a shield other planets — Olympus Mons on volcano is frequently Mars is the largest volcano found in followed by collapse of the the solar system. summit region — forming Olympus Mons rises 24 km from its a caldera (>1 km). base and measures 550 km A caldera is a circular across. depression where the

USGS By comparison, Earth's largest surface has slumped as volcano, Mauna Loa in Hawaii, lava has poured out from rises only 9 km high and measures the magma chamber Valles Caldera 120 km across. beneath. Note the caldera at the summit of Olympus Mons.

NASA Landsat 7 Landsat

Composite Volcanoes Mt. Shasta and Mt. Lassen in California Composite Volcanoes or represent the southern extent of the USGS Stratovolcanoes are large and of volcanoes that result commonly symmetric and are from subduction along the NW coast. composed of alternating lava flows and A smaller parasitic cone (Mt. ) can pyroclastic deposits. be seen on the flank of Mt. Shasta. They are produced by relatively viscous such as Mt. Lassen is a remnant of a much larger (intermediate between mafic and felsic) volcano (Mt. Tehama) and is one of only and are usually associated with two volcanoes to have historically erupted subduction zones. in the continental U.S. The photo shows Mt. Rainier in Washington. These are the most violent types of volcanoes. Examples of composite volcanoes include Mt. Fuji, Japan and the This video shows the 1915 eruption of Cascade range in N. California, Mt. Lassen. Oregon and Washington: Mt. Shasta, Mt. Hood, Mt. St. Helens, Mt. Rainier USGS Commonly, the summit of Cinder Cones the composite volcano may Cinder cones are small (<300 m), collapse at the end of a steep-sided (30-40°) and are violent eruptive cycle — composed of loose pyroclastic forming a caldera. Examples (literally fire particle) material. They of calderas include Crater represent a short volcanic episode Lake (OR) (top) and from start to finish and commonly

Aniakchak Caldera (AK) occur in groups (fields). Pu`u ka Pele USGS (bottom). They are predominantly basaltic (mafic) in composition).

Sunset Crater USGS

Cinder cones are common throughout California and the western U.S. NPS

II Plutonic Activity When magma is emplaced at depths and crystallizes, it forms a variety Volcanism is spectacular, but most magma is emplaced at depth in the of plutonic bodies. crust. After volcanism has ceased, erosion of the surface may expose these There are two types of igneous activities: buried plutonic bodies. • Volcanic Thus, by investigating these features that are now at the surface, we are • Plutonic glimpsing at the "plumbing system" for an ancient volcanic system. Keep in mind that these two processes occur simultaneously and involve the same materials. There is a vast "plumbing system" for volcanoes.

USGS USGS III Plate Tectonics and Igneous Activity Volcanoes occur within three tectonic zones: Looking at a map of the global distribution of volcanoes, it is apparent • convergent plate boundaries - subduction that they occur in certain tectonic environments. Most occur along plate • divergent plate boundaries - seafloor spreading at mid-ocean margins or boundaries. The figure below shows the distribution of some ridges of the Earth's major volcanoes. • interior of plates - hot spot volcanism (ex. Hawaii)

Igneous Activity at Spreading Centers The greatest volume of on the Earth is produced The figure along the oceanic ridge system. NASA shows the tectonic As plates pull apart, the pressure setting for of the underlying rocks is each zone lessened. The reduced pressure of results in partial melting of the volcanism. mantle. Lava works it way up to form new basalt seafloor. NOAA

USGS Submarine lava flows typically consist of pillow lavas. Most volcanoes occur associated with subduction zones (convergent margin). Seafloor spreading (divergent margin) is associated with a large amount of volcanic activity. There are a few volcanoes (Hawaii) that occur in the middle of tectonic plates and are not associated with plate boundaries (hot spot volcanism). Igneous Activity at Subduction Zones Magma is generated when water is driven out of the subducted plate. The buoyant water migrates upward where it causes partial melting of As a seafloor slab sinks into the USGS mantle at a subduction zone, the mantle wedge. (Magma forms by melting in the mantle and not by melting of rock occurs in the mantle melting of the subducted plate.) above the subducted plate. The magma The magma rises through the crust rises buoyantly where it may form volcanoes. through the crust. As it The subducted slab may diving migrates under continental crust forming a upward through continental arc. This is the origin of the crust, it USGS the Cascade and Andes volcanoes. changes Likewise, the subducted slab may composition dive under another oceanic plate (more later) and and form an island arc. This is the may accumulate origin of Japan, the Philippines, and in the upper the Aleutian Islands. crust in magma chambers.

Tanya Atwater, UCSB

Mt. Pavlof The chemistry of the magma USGS Note the ring of volcanoes that circles the Pacific Ocean. associated with stratovolcanoes is There are many subduction zones that border the Pacific and it is known higher in silica and water than as the Ring of Fire. basalt. This causes these volcanoes (andesite) to be more explosive than basalt volcanoes and represents some of the most dangerous volcanoes in the world.

Mt. Pinatubo Intraplate Igneous Activity This figure shows the location of hot spots. Note that hot spots occur Although most igneous beneath oceanic and continental crust — Africa seems to be activity occurs along plate particularly abundant in hot spot. boundaries, there are some Many of the these hot spots are easy to identify as lines of seamounts regions of igneous activities and volcanoes on the map of the seafloor. that occur within oceanic and continental plates (ex. Hawaii and Yellowstone). The explanation for these intraplate volcanoes was not adequately explain until the development of the theory of plate tectonics. There are plumes in the mantle where hot material is rising known as mantle hot spots. The mantle hot spots remain essentially stationary USGS but when the crust moves USGS over the hot spot, melting can occur resulting in volcanic activity.

This figure shows the Yellowstone Wheeling Jesuit University Classroom of the Future Hawaiian Islands and As North America seamount chain and the drifted southwest Emperor seamount chain. over the Yellowstone In addition, the age of hotspot, the each is listed. volcanism progressed Kilauea is currently northeast, beginning erupting and is located in northern Nevada directly over the hot spot. 16.5 million years However, the further the ago and reaching island or seamount is from Yellowstone National the hot spot, the older the Park 2 million years age. ago. A new Hawaiian USGS island/seamount, Loihi, is starting to form off the southeast coast of the Big A series of ancient caldera complexes have been recognized on the Island. These data are also excellent evidence in Snake River Plain - a flood basalt region. support of the theory of plate tectonics. IV Magma and Lava Composition of Igneous Rocks Generally, magma formation begins in the upper mantle by a variety of Felsic/Silicic different processes. Partial melting of the rocks of the upper mantle produce basalt (basalt is the most abundant rock on the Earth’s surface – seafloor). Intermediate Melting in the lower crust is generally associated with uprising magma Mafic from the mantle. Magma may change its composition as it ascends through the crust by a variety of processes. USGS A survey of igneous rocks indicates that there are many different types with different chemical compositions. Generally, the composition of igneous rocks

can be categorized by the SiO2 content. • Felsic composition rocks are rich in SiO2 and other elements (K and Na). They are generally poor in Fe, Mg, and Ca.

• Mafic composition rocks are poor in SiO2 and rich in Fe, Mg, and Ca. • Intermediate compositions are ‘intermediate’ in composition between felsic and mafic.

• Ultramafic compositions are the lowest in SiO2 and the richest in Fe, Mg, and Ca. Ultramafic magmas/lavas occurred in the distant past but do not occur today. National Taiwan Science Education Center

Viscosity Factors Affecting Viscosity The viscosity of lava is important because it affects how it behaves and 1. Temperature. As the temperature decreases, pahoehoe is directly related to the violence of a volcanic eruption. the viscosity increases. Intermediate and felsic The viscosity of a fluid is its resistance to flow: magmas/lavas are lower temperature than basalt. • low viscosity fluids are very fluid and mobile Lower temperature magmas are generally more • high viscosity fluids are very thick and sticky explosive. The transition from low viscosity pahoehoe lava (basalt) to high viscosity aa lava Low viscosity lavas such as basalt can (basalt) is do the cooling of the lava flow. USGS flow very easy over the surface of the Basalt magma 1000-1200°C aa earth and may form lava rivers and Low viscosity basalt Andesite magma 800-1000°C fountains. lava (Hawaii) and high magma 650-800°C viscosity andesite lava 2. Magma Composition. Magmas that are richer High viscosity lavas such as andesite (Mount St. Helens). in silica or quartz (SiO2) are generally more and rhyolite don’t flow easily and viscous and therefore more explosive. usually form crumbly rocky lava flows Temperature and composition are not really USGS that look like rock piles moving independent of one another. downhill. The viscous magma/lava can 3. Degree of Crystallization. A magma with significant crystallization of trap volcanic gases and commonly minerals is generally more viscous than a magma that is 100% molten. result in explosive eruptions. As an The degree of crystallization is not independent of temperature and initially low viscosity basalt lava flow composition. cools, it can also become very viscous. Importance of Dissolved Gases V Types of Lava Flows At depth in the Earth nearly all magmas contain dissolved gas. The The image shows a typical gases in magma are mostly water vapor and CO . 2 fountaining eruption at Kilauea in The amount of gas in a magma is also related to the chemical Hawaii. composition of the magma. Felsic magmas generally have higher gas contents than mafic magmas. The style of eruption (explosive or As the magma nears the surface, gases come out of solution and their more fluid) depends on a number of volume may increase hundreds of time the original volume. This is factors including lava composition similar to opening a bottle of pop. and volatile content. USGS Magmas contain 1 - 6 weight% gas. Basaltic (mafic) lava flows are The gas content strongly affects the mode of generally very fluid. eruption — when the pressure is released during an eruption, the gases begin to escape and expand. Pyroclastic material (ash, dust, bombs) are produced by the violent eruption of gas-laden viscous magmas.

Generally, the viscosity of lava increases The video shows a typical aa flow and the lava flow changes its character as in Hawaii. it cools and degasses. As the lava cools as it flows, its When basaltic lava first erupts, it is very viscosity increases until the flow fluid and flows like a viscous fluid. consists of rough, jagged blocks This type of lava flow is known as that push forward known. This pahoehoe and has a smooth to ropey type of lava flow is known as aa. surface.

USGS

USGS Sometimes lava flows are The photo on the right shows submarine and occur on the pillow on the seafloor ocean floor. The style of near Hawaii. eruption is different than the subaerial flows. As lava erupts underwater, it flows in lobate forms that pile on top of each other forming NOAA lava pillows. NURF

The photo to the left shows pillow lavas that formed in a submarine environment. Pillow lavas are common in California.

AGU

The video/animation shows the The eruption of a composite Mayon Volcano, Philippines May 18, 1980 eruption of Mt. St. volcano can result in the Helens. production of a very hot cloud Preceding this major eruption, of gas and ash (up to 700°C) magma had moved upward into that is heavier than air. As it the volcano swelling its flanks. The moves out of the volcanic unstable slopes had visible bulges vent, it moves downslope at from the inflation of magma. speeds of up to 80 km/hour. When magma moves, it causes This is known as an ash flow USGS or . Mt. St. Helens small earthquakes (harmonic YouTube tremors). In this movie, you can These types of eruptions can see a landslide (in slow motion) move rapidly up to 10’s of km due to an harmonic tremor. from their source and are The large landslide resulted in the decompression of the magma inside utterly devastating. the volcano and it explosively released its gas. This is similar to opening a shaken bottle of soda pop. The explosive eruption sent tons of ash and debris into the air and blasted one flank of the mountain. USGS